Despite a long research history in the field of wind turbine aerodynamics, horizontal axis wind turbine (HAWT) blade's root flow aerodynamics is among the least understood topics. In this thesis work, a detailed investigation of the root flow is performed to gain a better insight into the features of this particular flow region and their influence on the overall air flow. Two- and three-dimensional flow analyses of a HAWT blade are performed for axial in flow conditions, with both experimental and numerical approaches. In the experiments, a stereoscopic particle image velocimetry (SPIV) setup is used to measure velocity in the near wake region at different azimuth angles and around the blade at different radial positions. This experimental setup allows measuring three velocity components on 2D planes which can be used to construct three-dimensional flow fields. With this approach, a detailed description of the flow-field in the root region is obtained and 3D visualizations are presented. Further analysis of the velocity fields allows illustrating and understanding the physics of the formation of the root flow structures for different blade geometries and their evolution in the blade's near wake for different blade tip speed ratios. The effect of the root vortex on the blade's root flow and in the near wake region is studied. In particular, the experimentally-observed spanwise flow in the blade's outer flow region (outside the boundary layer of the blade) questions the two-dimensional flow assumption of the classical momentum theory. The velocity fields are also used to deduce the loads on the blade through the calculation of the momentum change in the fluid. In addition to the analysis of the experimental results, also comparisons with numerical simulations from Blade Element Momentum (BEM) and Computational Fluid Dynamics (CFD) are made. The (Open Foam) CFD results are validated by comparing the computed velocity fields with the PIV results and a good agreement is obtained. The comparison of the load predictions from the numerical and the experimental methods also shows a very good agreement, which brings confidence about the capability of these numerical methods to estimate the forces along the blade. This thesis has contributed to narrowing the knowledge gap in the field of HAWT blade's root flow aerodynamics by: (i) providing a solid experimental database of root flow velocities and vortical structures; (ii) investigating the existence and hence the role of the root vortex; (iii) studying the spanwise flow over the blade's surface and hence identifying the three-dimensionality of the flow in the outer flow region; (iv) comparing the experimental and numerical results to study and explain the physics of the root flow; (v) demonstrating that with advanced numerical tools realistic and complicated root flow details can be simulated.